1. Field of the Invention
The present invention relates to an integrated type optical waveguide device, and more particularly, to the technique to improve the performance of the integrated type optical waveguide device with a hybrid structure.
2. Description of Related Art
Conventionally, as one of integrated type optical waveguide devices, a variable optical attenuator (VOA) is known using a waveguide type Mach-Zehnder circuit, as shown in FIG. 1. The variable optical attenuator is composed of a phase shifter section 10, an input section 20 and an output section 30. An interference region is formed in the input section 20 and the output section 30. An optical waveguide 40 is formed in each section. An optical Y-branching or Y-combining circuit or a directional coupler is formed in the interference region to function as a splitter or a combiner.
In the variable optical attenuator, an optical signal inputted from an optical input fiber 21 is branched into two optical signals by the Y-branching circuit which is formed in the input section 20. The two optical signals are sent to the phase shifter section 10. In the phase shifter section 10, the phase of each optical signal is shifted and the phase-shifted optical signals are sent to the output section 30. The two phase-shifted optical signals having different phases are combined by the Y-combining circuit and are outputted to an optical output fiber 31. At the combination, the optical signal is attenuated. An attenuation quantity of the optical signal is controlled based on a quantity of phase shifted by the phase shifter section 10.
The optical waveguide used in the above variable optical attenuator is, as a whole, a single material waveguide such as a silica system waveguide, a polymer waveguide, LiNbO3 (lithium niobate, and hereinafter sometimes referred to as “LN”) waveguide, a semiconductor waveguide. For the phase control, the LiNbO3 waveguide or the polymer waveguide using electro-optical effect (EO effect) or the silica-based waveguide or the polymer waveguide using thermal-optical effect (TO effect) is used in many cases.
By the way, when a variable optical attenuator is formed using a diffusion-type optical waveguide which is formed through thermal diffusion into the LN substrate, the control rate can be made high because LN has the electro-optical effect. Also, the power consumption is very small because the optical signal can be controlled using the electric field generated in response to application of a voltage.
On the other hand, there are the following problems in the variable optical attenuator that a diffusion-type optical waveguide is formed on the LN substrate. First, the refraction indexes are different between the diffusion-type optical waveguide and the optical fiber because the material of the diffusion-type optical waveguide is different from that of and the material of the optical fiber. Also, the coupling loss between the optical fiber and the diffusion-type optical waveguide is large because the cross section of the diffusion-type optical waveguide is different from that of the optical fiber section. Second, when the ability to confine light is small so that a bending loss is large because the refraction index difference is small in the diffusion-type optical waveguide. As a result, the optical device cannot be made small because it is not possible to reduce the radius of curvature of the optical waveguide. Third, the optical polarization dependence is caused because a refraction index distribution and a stress distribution in the optical waveguide are asymmetry in a Y-branching circuit and a directional coupler which are formed as an interference region.
On the other hand, when a variable optical attenuator is formed using an embedded type optical waveguide formed by depositing silica-based material on a silicon substrate, there are the following advantages. First, the refraction index of the optical waveguide is same as that of the optical fiber, because the optical fiber is formed of silica-based material. In this case, the coupling loss can be made very small due to the refraction index difference. Also, because the silica-based material is easy in processing, the section shape of the optical waveguide can be made same as that of the optical fiber, resulting in reduction of the coupling loss. Second, the adjustment of the refraction index difference is easy in the silica-based waveguide. Also, it is possible to increase the ability to confine light. Therefore, the bending loss can be made small. Third, because it is possible to make the refraction index distribution symmetrical in the silica-based waveguide, it is possible to reduce the optical polarization dependence, when the Y-branching circuit and the directional coupler are formed of the silica-based material.
On the other hand, in case of a phase shifter, thermo-optical effect must be used because the electro-optical effect cannot be used. The phase shifter using the thermo-optical effect controls a phase shift quantity through the heating. Therefore, the control speed cannot be made high. Also, the power consumption is very large, compared with the phase shifter using the electro-optical effect. Therefore, it is difficult to form an optical device with multiple stages, especially. Also, because the control using the thermo-optical effect is easy to undergo influence of environment temperature, elements influence each other due to difficulty of heat confinement so that the characteristic of the device degrades, especially in an array structure.
The technique for restraining optical polarization dependence is disclosed in Japanese Laid Open Patent Application (JP-A-Showa 62-36631) titled “waveguide-type light modulator”. In the waveguide-type light modulator, input light is separated into TE mode light and TM mode light using polarization light splitter (PBS), phase modulation is carried out individually, and then the phase-modulated lights are combined again using the optical polarization combiner. According to the waveguide-type light modulator, the input light is split for every mode, and modulated, so that optical polarization dependence is restrained. However, the waveguide-type light modulator is complicated in the structure because trimming is carried out using a quarter wavelength board.
In conjunction with the above description, a waveguide type optical switch is disclosed in Japanese Laid Open Patent Application (JP-A-Showa 64-63934). In this conventional example, a coupling waveguide is provided between an optical switch section and a coupling optical fiber, and the coupling waveguide is formed of a material smaller in a coupling loss than the optical switch.
Also, an optical switch is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 5-297420). In this conventional example, the optical switch is composed of a plurality of optical waveguides formed on a substrate and a 2-input and 2-output directional coupler. An element with a complete reflection function is provided for the waveguide on the one output side of the directional coupler and an element with a complete reflection function and a phase modulation element are provided for the waveguide on the other output side of the directional coupler.
Also, a composite light circuit is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 7-20413). In the composite light circuit of this conventional example, a first light circuit having a waveguide for branching or combining an optical signal is coupled to a second light circuit which has a waveguide for modulating or filtering the optical signal at high speed, through a refraction index adjustment region. In the composite light circuit, a quartz waveguide type light circuit is used as the first light circuit. Also, a lithium niobate system waveguide type light circuit is used as the second light circuit. The composite light circuit can handle eight or more of waveguides at the same time at a high speed.
Also, an optical module is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 10-160977). In this conventional example, the optical module is composed of an optical hybrid circuit and an optical passive circuit. The optical hybrid circuit is composed of an optical waveguide formed on a substrate, and optical function elements formed on the substrate while keeping optical coupling with the optical waveguide. The optical passive circuit is composed of an optical waveguide formed on another substrate, and is directly connected with the optical hybrid circuit in one end and is connected with an optical fiber in the other end.
Also, a laser oscillation method is disclosed in Japanese Laid Open Patent Application (JP-A-2000-22246A). In this conventional example, reflection sections are provided for ends of an optical fiber in which laser ions are doped, and the doped ions are excited by an excitation laser section and a laser beam with multiple wavelength components is generated. Here, a laser beam outputted from the optical fiber is split for every wavelengths and a plurality of split laser beams are reflected by the reflection sections, respectively.